Affiliation: Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York, United States of America.

ABSTRACTEvolutionarily conserved mitogen activated protein kinase (MAPK) pathways regulate the response to stress as well as cell differentiation. In Saccharomyces cerevisiae, growth in non-preferred carbon sources (like galactose) induces differentiation to the filamentous cell type through an extracellular-signal regulated kinase (ERK)-type MAPK pathway. The filamentous growth MAPK pathway shares components with a p38-type High Osmolarity Glycerol response (HOG) pathway, which regulates the response to changes in osmolarity. To determine the extent of functional overlap between the MAPK pathways, comparative RNA sequencing was performed, which uncovered an unexpected role for the HOG pathway in regulating the response to growth in galactose. The HOG pathway was induced during growth in galactose, which required the nutrient regulatory AMP-dependent protein kinase (AMPK) Snf1p, an intact respiratory chain, and a functional tricarboxylic acid (TCA) cycle. The unfolded protein response (UPR) kinase Ire1p was also required for HOG pathway activation in this context. Thus, the filamentous growth and HOG pathways are both active during growth in galactose. The two pathways redundantly promoted growth in galactose, but paradoxically, they also inhibited each other's activities. Such cross-modulation was critical to optimize the differentiation response. The human fungal pathogen Candida albicans showed a similar regulatory circuit. Thus, an evolutionarily conserved regulatory axis links metabolic respiration and AMPK to Ire1p, which regulates a differentiation response involving the modulated activity of ERK and p38 MAPK pathways.

Mentions:
Protein glycosylation is an oligosaccharide modification of proteins that occurs in the endoplasmic reticulum (ER) and Golgi apparatus [108]. Defects in protein glycosylation trigger a global response that involves the action of several MAPK pathways, including the filamentous growth [10], [60] and HOG pathways [63], [64]. Comparative RNA seq analysis identified HOG pathway targets induced by treatment with tunicamycin, an inhibitor of N-linked glycosylation (Fig. 1, B and C; Table S1). To further explore the HOG and filamentous growth pathway response to glycosylation deficiency, a conditional mutant, pmi40-101[60], was used that is defective for an early step in N- and O-linked glycosylation [109], [110]. Growth of the pmi40-101 mutant in media lacking mannose induces its glycosylation defect and showed elevated HOG and filamentous growth pathway activity (Fig. 4A). Defects in O-linked glycosylation also modestly activated the HOG pathway (Fig. S5A). In response to glycosylation deficiency, HOG pathway activation did not require Snf1p (Fig. S5B), which is consistent with the idea that Snf1p regulates the HOG pathway by the de-repression of glucose-repressed genes.

Mentions:
Protein glycosylation is an oligosaccharide modification of proteins that occurs in the endoplasmic reticulum (ER) and Golgi apparatus [108]. Defects in protein glycosylation trigger a global response that involves the action of several MAPK pathways, including the filamentous growth [10], [60] and HOG pathways [63], [64]. Comparative RNA seq analysis identified HOG pathway targets induced by treatment with tunicamycin, an inhibitor of N-linked glycosylation (Fig. 1, B and C; Table S1). To further explore the HOG and filamentous growth pathway response to glycosylation deficiency, a conditional mutant, pmi40-101[60], was used that is defective for an early step in N- and O-linked glycosylation [109], [110]. Growth of the pmi40-101 mutant in media lacking mannose induces its glycosylation defect and showed elevated HOG and filamentous growth pathway activity (Fig. 4A). Defects in O-linked glycosylation also modestly activated the HOG pathway (Fig. S5A). In response to glycosylation deficiency, HOG pathway activation did not require Snf1p (Fig. S5B), which is consistent with the idea that Snf1p regulates the HOG pathway by the de-repression of glucose-repressed genes.

Affiliation:
Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York, United States of America.

ABSTRACTEvolutionarily conserved mitogen activated protein kinase (MAPK) pathways regulate the response to stress as well as cell differentiation. In Saccharomyces cerevisiae, growth in non-preferred carbon sources (like galactose) induces differentiation to the filamentous cell type through an extracellular-signal regulated kinase (ERK)-type MAPK pathway. The filamentous growth MAPK pathway shares components with a p38-type High Osmolarity Glycerol response (HOG) pathway, which regulates the response to changes in osmolarity. To determine the extent of functional overlap between the MAPK pathways, comparative RNA sequencing was performed, which uncovered an unexpected role for the HOG pathway in regulating the response to growth in galactose. The HOG pathway was induced during growth in galactose, which required the nutrient regulatory AMP-dependent protein kinase (AMPK) Snf1p, an intact respiratory chain, and a functional tricarboxylic acid (TCA) cycle. The unfolded protein response (UPR) kinase Ire1p was also required for HOG pathway activation in this context. Thus, the filamentous growth and HOG pathways are both active during growth in galactose. The two pathways redundantly promoted growth in galactose, but paradoxically, they also inhibited each other's activities. Such cross-modulation was critical to optimize the differentiation response. The human fungal pathogen Candida albicans showed a similar regulatory circuit. Thus, an evolutionarily conserved regulatory axis links metabolic respiration and AMPK to Ire1p, which regulates a differentiation response involving the modulated activity of ERK and p38 MAPK pathways.